Aether can be portraited as the Traveler nor the Abyss Prince, Dainsleif is often aware of Aether as they are enemies as the Abyss Prince if Lumine were to be the Traveler.
There is a video called the Teyvat Chapter Storyline Preview, at the end of its video. While Dainsleif was narrating, he tells Aether; "Defeat me, command me to step aside, show me that you are worthier than I to rescue her.". Saying that once Aether defeats him and shows him he is worthier than him to rescue her, referring that 'her' to Lumine.
Furthermore, once the video fades, about to finish. Dainsleif finishes off with saying, "My memory has all but faded completely... But I will always remember how much she too, love those flowers.". Becoming more noticeable that Dainsleif definitely knew Lumine's existence, and that, Dainsleif is confirmed to be aware of Lumine's appearance as he is shown to know Lumine a lot in the Bough Keeper: Dainsleif Quest as they were formerly allies but until the destruction of Khaenri'ah, which they became enemies.
Bough Keeper: Dainsleif
Starting on Aether being the Traveler. In Act 1 of Bough Keeper: Dainsleif, Aether meets Dainsleif at the Tavern, Aether introduces himself as from the Adventurers Guild, and as the Honorary Knight. Yet, Dainsleif didn't respond, until Aether tells that he's a Traveler. Into which, Dainsleif finally responded. Dainsleif questions Aether onto why he's traveling, Aether answers him that he's looking for his lost relative which is of course, Lumine. Dainsleif then offers Aether to sit with him, Dainsleif introduces himself and asks if he needs something from him, Aether tells him that he was inviting him to adventure with them.
Dainsleif agrees upon the invitation meaning that he will joins on Aether's adventures, but Dainsleif tells that he requires a payment, it was Aether to give him 500 mora which it isn't that much, yet Dainsleif told Aether to answer that questions that he will give. Aether then agree and Dainsleif then finally started with the questions. Starting off with the first question, Dainsleif asks Aether who in his view was the key to end the crisis in Mondstadt, this actually leads to the player to decide as Aether is the main character, along with Lumine as well. On the second question, Dainsleif asks who will defend Liyue Harbor in the future, leading again, to the players to decide. It was then the final question, Dainsleif asks who is more important, the ones with vision, the ones with not, or no one.
After finishing all the questions, when the player picks "The unity of Mondstadt's people gave us victory" and "Everyone in Liyue Harbor". Dainsleif will say " As I thought... you really are similar to her.", but if you didn't pick neither of those, Dainsleif will say "It seems that you're different from her...".
Continuing on, Aether and Dainsleif then went to the Temple of Falcon to defeat enemies on there, going the Wolvendom, Aether then defeats some enemies again. And lastly, they then went to Stormterror's Lair, when going deeper into Stormterror's Lair, Aether comes across some enemies again said, Aether defeated them. While talking, Aether then felt something on a cliff, thinking it would be the Abyss Order, they then went to the cliff and saw a Ruin Guard's footprints. Aether then examined the dandelion until he had a flashback where Lumine was captured which made Aether seemingly willing to find her until Dainsleif stopped him and tells that the important thing in his journey is the find her as his journey has only yet, to begin. Dainsleif then left, leaving into that being the finishing of Act 1 in Bough Keeper: Dainsleif.
Moving on to the next Act, 'We Will Be Reunited'. After Aether leaves the ruins, he meets Dainsleif again. Aether tells Dainsleif about the strange statue and the deceased thief that he have encountered in the ruins. Dainsleif says that it's a rare opportunity and tells Aether to catch up with him, helping him to solve more mysteries within the Abyss. The two looks for more traces within the Abyss Order, till Dainsleif tells about Khaenri'ah. Aether told Paimon that he has memories of Khaenri'ah, so Dainsleif told that he will not pry his secret because he didn't pry his. Dainsleif asks Aether, what was the Khaenri'ah that he saw.
Dainther isn't that popular among the Genshin Impact fandom, Dainther is often overshadowed by Xiaother and DainKae. Which are the two most popular ships. On Archive of Our Own, there are currently 20 fanfictions, depicting the two. There is also a theory going around in the fandom that Dainsleif is Aether and Lumine's brother. Another theory is, Dainsleif is the future Aether due to their similar appearances that the two have. There are more theories between the two as the lore continues.